This invention relates to controlling image sharpening of digital electronic images.
Processes such scanning, printing, and resampling often introduce blurring of electronic images. Sharpening engines are computer programs (or program modules) that can be used to correct such blurring. For example, unsharp masking is a conventional sharpening technique that is widely implemented in graphics programs as a tool that a user can select and apply to sharpen edges in a blurred image. Typically, an unsharp masking filter operates by locating a pixel that differs from surrounding pixels by a specific threshold (i.e., an edge pixel), increasing the brightness when the edge pixel is on the lighter side of the edge, and decreasing the brightness when the edge pixel is on the darker side of the edge. The brightened edge pixels on the lighter side of an edge form a bright line along the edge and are typically called the “white line.” Similarly, the darkened edge pixels on the darker side of an edge form a dark line along the edge and are typically called the “black line.” Generally, these lines are collectively referred to as “contrast lines.”
With unsharp masking, the sharpening engine typically relies on three sharpening parameters to specify how an image is to be sharpened: the radius, the threshold, and the amount of sharpening. The radius specifies the width of the white and black lines. In principle, the radius can be represented in terms of pixels or in terms of a measured area, in which latter case the value is resolution independent. A low radius value produces narrow contrast lines, while a high value produces wide contrast lines. The threshold indicates the amount of difference required for an edge to be recognized between any two pixels. A low threshold value would cause a larger portion of the pixels of an image to be modified by sharpening than would a higher threshold value. The third sharpening parameter, the amount sharpen, determines the percent by which sharpening increases the contrast between pixels that have been selected for sharpening.
The proper operation of a sharpening process may depend on the resolution of the image being sharpened. Generally, sharpening parameters are set with reference to a particular output size, and therefore resolution, of an image. For example, if a 400×400 pixel image of a balloon is sharpened for printing to an 7×10 inch area, but actually printed to 3.5×5 inch area, contrast lines would form an undesirable halo around the edge of the balloon.
The proper operation of a sharpening process may also depend on matching the process parameters to a particular output device. Generally, for a given image, different output device types will not produce exactly the same output. For example, continuous tone and halftone printers will produce renditions of the same image that look different. Thus, sharpening an image for printing from one type of printer will not necessarily produce the correct level of sharpened image appearance on another output device.